Of particular interest in the following discussion is the manufacture of lithium containing glasses. As is appreciated by those skilled in the art, lithium containing glasses are usually used as a substrate to make ion exchanged strengthened glass. One type of lithium containing glass is disclosed in U.S. Pat. No. 4,156,755 (“hereinafter also referred to as “USPN '755”), which patent is incorporated herein by reference.
In general, iron is not a required ingredient to make lithium containing glass for the ion exchange process, however, small amounts of iron are usually present in the lithium containing glass as an impurity in the glass batch ingredients, or the iron is added to the glass batch materials to provide a lithium containing glass having desired properties, e.g. but not limited to optical and/or colored properties. Total iron oxide content as Fe2O3 in commercial glasses depends on the product requirements but are commonly in the range of 50-1200 parts per million {hereinafter also referred to as “PPM”} or 0.005-0.12% of the total iron by weight on the oxide basis (hereinafter referred to as “percent by weight” or “wt %”) for what are considered clear glass compositions. More particularly, the addition of iron can be made as ferrous iron (FeO) or as ferric iron (Fe2O3). During the melting of the glass batch materials, equilibrium is reached between the ferric form of iron (Fe+++) and the ferrous form of iron (Fe++) with about 25-30 wt % of the iron in the ferrous form (Fe++) and 70-75 wt % of the iron in the ferric form (Fe+++). The ferric oxide, Fe2O3, is a strong ultraviolet radiation absorber and operates as a yellow colorant in the glass, and the ferrous oxide, FeO, is a strong infrared radiation absorber and operates as a blue colorant in the glass. Of particular interest in the present discussion is the ferrous oxide, FeO.
In the instance when a glass sheet, for example but not limiting to the discussion, a lithium containing glass sheet (hereinafter also referred to as a “lithium glass sheet”) is to be heated, e.g. but not limiting to the discussion, prior to bending and shaping, the composition of the lithium glass sheet usually includes ferrous oxide (FeO) in the range of 0.02 to 0.04 wt %, and the lithium glass sheet has a redox ratio (discussed in detail below) of 0.2 to 0.4. In the instance when a lithium containing glass (hereinafter also referred to as “lithium glass”) is to be used in the practice of the invention as a viewing window for infrared equipment, e.g. but not limited to, infrared night goggles, or as components of transparent armor or aerospace windows, the ferrous oxide is preferably in the range of 0.001 to 0.010 wt %, and the lithium glass has a preferred redox ratio in the range of 0.005 to 0.10. The wt % of ferrous oxide is higher for the lithium glass sheet to be heated to increase the absorption of the infrared wavelengths to decrease the heating time of the lithium glass sheet to reach the bending temperatures. The wt % of the ferrous oxide is low for the lithium glass to be used for a viewing window for infrared equipment in accordance to the teachings of the invention to increase the percent transmittance of infrared energy through the viewing window.
One of the drawbacks with going from a Campaign making a high infrared absorbing (hereinafter also referred to as “HIRA”) lithium glass to a Campaign making low infrared absorbing (hereinafter also referred to as “LIRA”) lithium glass of the invention, and/or going from a Campaign making a LIRA lithium glass of the invention to a Campaign making HIRA lithium glass, is the quantity of glass produced during the period starting at the end of one Campaign, e.g. the end of the Campaign to make HIRA lithium glass, and ending at the start of the next Campaign, e.g. the start of the Campaign to make LIRA lithium glass that meets the specifications for LIRA lithium glass or HIRA lithium glass. The glass that is out of specifications for use as LIRA lithium glass and HIRA lithium glass is usually scrapped or used as cullet. It can now be appreciated by those skilled in the art that discarding the glass made during the change from one Campaign to another Campaign is costly due to the relatively high batch cost for lithium glass and to the time wasted making unusable glass or glass of marginal quality.
It is advantageous, therefore, to provide a method of minimizing or eliminating the drawbacks associated with changing from a Campaign making useable HIRA lithium glass or useable LIRA lithium glass to a Campaign making useable LIRA lithium glass or useable HIRA lithium glass, respectively.